1. Field of the Invention
The present invention relates to a magnetic recording/reproducing apparatus and, more particularly, to a magnetic recording/reproducing apparatus capable of decreasing an error rate in recording and/or reproduction of a digital signal.
2. Description of the Related Art
Recently, digital VTRs for digitizing a video signal and recording/reproducing the digital video signal to obtain a high quality image are becoming popular. For broadcasting systems, digital VTRs based on the D1 and D2 formats have already been commercially available. In a digital VTR of this type, since the amount of data to be recorded is very large, head each having a small gap and a small depth are employed to increase the recording density.
Generally, a magnetic head- is gradually worn by contact with a recording medium. As a result, the depth of the gap formed in the head is gradually decreased. When the gap depth of the head is decreased, the strength of the magnetic field of the gap portion of the write head is relatively increased, and the reproduced output voltage characteristic curve shown in FIG. 10 with respect to the recording current changes from a broken curve a to a solid curve b. As a result, as the wear progresses, the value of the recording current is deviated from an optimum recording current value. At this time, the waveform of the reproduced pulse signal is changed, like from the broken curve to the solid curve, as shown in FIG. 10, and the peak of an FWHM (Full Width Half Maximum) is decreased, like the solid curve in FIG. 10, to widen the curve in the right-and-left direction. As a result, an ISI (Inter Symbol Interference) tends to occur and increases the error rate. In order to solve this problem, wear of the head may be anticipated in advance and the recording current value may be periodically adjusted. With this method, however, the maintenance becomes cumbersome.
Generally, a reproducing system of a digital magnetic recording/reproducing apparatus, e.g., a digital VTR, has an equalizer for equalizing the waveform of a reproduced signal. The influence of the "deviation (i.e. separation)" of the recording current from the optimum value may be compensated for by using the equalizer. Equalizers of this type are classified into a fixed equalizer having fixed equalizing characteristics and a variable equalizer having equalizing characteristics which are adaptively controlled in accordance with the residual distortion of the waveform of the reproduced signal.
When a fixed equalizer is used, in order to compensate for the influence of the deviation of the recording current, the equalizing characteristics may be periodically adjusted. However, this maintenance operation is cumbersome just like the manual adjustment of the recording current described above. As a variable equalizer, an equalizer comprising linear filters, e.g., transversal filters, is well known. When such a variable equalizer is used, however, a large number of filter taps are needed in order to remove the distortion from a reproduced signal waveform, which distortion is caused by the deviation of the recording current from the optimum state, resulting in an increase in circuit size of the equalizer.
Even if the reproduced signal waveform is appropriately equalized by a fixed or variable equalizer, since the recording current value itself stays deviated from the optimum value, the S/N ratio of the reproduced signal is decreased.
When wear of the magnetic head progresses to reach a minimum allowable gap depth while the magnetic recording/reproducing apparatus is used, signal recording and reproduction failures occur. Especially, in an industrial VTR constantly requiring a high image quality, once the minimum allowable gap depth is reached, the head must be immediately replaced. Accordingly, the time of the reaching of the minimum allowable gap depth must be known in advance in order to perform head replacement at an appropriate timing. Conventionally, it is difficult to know the time a minimum allowable gap depth will be reached until wear of the head progresses to considerably degrade the image quality. Therefore, an average use time until the head reaches a minimum allowable gap depth is expediently given as the "service life of the head". The user replaces an old head with a new head when the service life of the head is estimated to have expired on the basis of the head use time.
In fact, however, the service life of the head largely depends on the environmental temperature, humidity, and the like. Sometimes the user replaces the head to avoid trouble even if the head can still be sufficiently used with a large head depth, thus wasting the head.
A conventional method of detecting a distortion in a reproduced signal is taught by, e.g., R.W. Lucky "Techniques for Adaptive Equalization of Digital Communication Systems", BSTJ, 45, 2, pp. 255-286 (1966). That is, a difference between a reproduced signal from a read head and an identified reproduced signal obtained by data-identifying the reproduced signal is defined as an error signal, and a value obtained by correlating or mutually relating the error signal and the data strings of the reproduced signal for a predetermined string length is determined as the "evaluation value of the distortion" of the reproduced signal. In this case, the positive or negative sign of the evaluated distortion value represents an increase or decrease of the distortion.
The method of manually re-adjusting the recording current value in order to remove the distortion in the reproduced signal waveform, as described above, results in cumbersome maintenance.
The method of compensating for the influence caused by wear of the head by utilizing the fixed equalizer of the reproducing system similarly requires manual adjustment of the equalizing characteristics. When a variable equalizer is used to compensate for the influence caused by wear of the head, the circuit size of the equalizer is increased.
In addition, even when either a fixed or variable equalizer is used, the recording current itself cannot be optimized, and the S/N ratio of the reproduced signal is decreased by wear of the head.
Furthermore, conventionally, the time to replace the heads cannot be precisely known by a user.